In his book Kant: A Biography (Chapter: "“All-Crushing” Critic of Metaphysics"), Manfred Kuhen gives a description of Kant’s view of the argument that Hume is offering. Kant believed that Hume’s argument consists of the following:
1) Assume the causal relation to be rational.
(2) If a relation is rational, it can be thought a priori and on the basis of concepts.
(3) For objects to be causally related, they must stand in a necessary relation, such that if one object is posited, the other one must also be posited.
(4) It is impossible to see by reason alone how the existence of one object necessitates the existence of another.
(5) Therefore, "it is wholly impossible to think such a conjunction a priori and out of concepts."
(6) Therefore, the causal relation is not rational.
7) Therefore, it is impossible to understand "how the concept of such an a priori connection can be introduced."
(8) Therefore, it must have some other source or sources, and the most reasonable ones are imagination and custom.
(9) But imagination and custom can produce only "subjective" necessity.
(10) Metaphysics requires necessity based on intersubjectively valid concepts.
(11) Therefore, metaphysics is impossible.
Kant thinks that the argument ending with (6) as a conclusion is sound. Hume "proved," he says, "irrefutably: that it is wholly impossible for reason to think such a conjunction a priori and out of concepts." What Kant does not accept is (7) and the conclusions founded upon it. He cannot, if only because this would show that the science of metaphysics is impossible. In order to save the science of metaphysics, or to show how it is possible, he must show how it is possible to introduce the concept of such a connection a priori. There is no reason for Kant to accept (7), in any case. From the fact that the causal relation cannot be shown by reason to be a priori, it does not follow that it cannot be shown to be a priori in some other way, just as it does not follow from the fact that I cannot determine the smell of an open sewer by sight that I cannot determine its smell in any other way. Thus Kant thinks he can make a plausible case that the causal connections have "their origin in pure understanding." Thus he argues that
(12) It is possible to introduce the concept of a priori connections by deducing them from the pure understanding.
One can make a distinction between "local skepticism," or a skepticism that relates only to a certain class of propositions, and "universal skepticism," or a skepticism that involves the doubting of the justifiability of any knowledge claim. Kant believes that Hume essentially establishes a form of "local skepticism," with "universal skepticism" being a hasty conclusion founded upon the former. Moreover, Kant does not see Hume as denying the existence of necessary synthetic judgments, but only as denying a certain way of justifying them. So Kant thought that he needed to give only a limited answer to Hume. All he had to do was justify synthetic a priori judgments, whose existence was admitted by Hume.Manfred Kuhen also points out that among his friends, Kant was known as the German Hume. Kant did not object to being regarded as close to Hume.